The reason for the stair-stepping of the OC Hierarchy is due to the fact that the next available level of multiplexing ('muxing") of lower-level circuits is usually 4: (4) OC-3s = (1) OC-12, and (4) OC-48s = (1) OC-192.

This muxing scheme is usually dictated by the equipment manufacturers and is pretty much an adopted standard in the Telecom industry - hence the lack of the lesser-common bandwidth aggregations like OC-9, OC-18, etc. The only exception is the OC-3, which was needed to allow the upper-level hierarchy to work. Hope this tidbit of info helps in the future!